Showing papers on "Superplasticity published in 1988"

Low-temperature deformation mechanisms and their interpretation

Abstract: Low-temperature deformation is characterized by heterogeneous strain in which the bulk of the material clearly retains its primary texture. Deformation is by grain-scale crystal plasticity, rotation, fracture, and pressure solution, and by transgranular mechanisms that crosscut numerous grains. The important low-temperature crystal-plastic features are twin lamellae, deformation bands, and undulatory extinction. Subgrain formation by recrystallization or crystal-plastic strain of more than 15% marks the upper limit of the low-temperature regime. Grain rotation may produce foliations in soft sediments or rocks. Microscopic to mesoscopic kinks and crenulations of bedding occur in soft clay and shale. Transgranular features include Luders' bands, cooling and desiccation cracks, joints, extension-fracture cleavage, clastic dikes, mineral-filled veins of several types, recrystallization/replacement veins, vein arrays, boudins, faults, stylolites, slickolites, solution cleavages that range from widely spaced to slaty and pencil cleavage. Pressure fringes form adjacent to relatively rigid grains and have fabrics analogous to those in veins. Faults include conjugate fault pairs (Andersonian faults) multiple simultaneous conjugates (Oertel faults), and Riedel shear-zone configurations. The sense of fault displacement is determined from bends, steps, trails, tails, and feather fractures. Superplasticity, especially if aided by diffusion in grain-boundary water, might be important at low temperatures. Fault textures are diagnostic of the environment of deformation but have yet to be uniquely correlated with the presence or absence of earthquakes. Riedel shears and pseudotachylite may form in earthquake source regions, although pseudotachylite is evidently rare in brittle fault zones. The best indicators of stress magnitudes are the critical' resolved shear stress for deformation twinning and the presence of tensile fractures. Strain magnitudes and stress and strain tensor orientations can be determined with a variety of methods that are based on mechanical twins, platy grain orientation, grain center distribution, and fault geometry and slip directions. Different deformation mechanism associations, expressed by the partitioning of the total strain into different mechanisms, are related to the ductility and environment of deformation. Deformation fronts separating different mechanism associations are defined on the basis of changes in the crystal-plastic component of strain.

Superplasticity of TZP/Al sub 2 O sub 3 composite

Abstract: The superplastic behavior of the composite Y-TZP/Al{sub 2}O{sub 3} (20 wt%) in uniaxial tension has been evaluated. Large elongation (>200%) indicated superplasticity. The stress exponent and activation energy of thee composite were found to be the same order with those of Y-TZP. The flow behavior of the composite containing Al{sub 2}O{sub 3} grains could be described by a rheological model as a non-Newtonian flow modified by the second-phase grains. The cavitation damage and the creep crack growth could be reduced by keeping the strain rate low enough that a specimen elongated 100% at elevated temperature maintained a strength of 1,800 MPa at room temperature.

An investigation of grain boundary sliding in superplasticity at high elongations

Abstract: Experiments were performed on the superplastic Zn-22% Al eutectoid alloy to determine the contribution of grain boundary sliding at both low (35%) and high (∼235%) elongations. The tests were conducted at two different strain rates in the superplastic Region II, and the results show that, within the accuracy of the measurements, there is a large sliding contribution at both elongations. By taking detailed measurements of both the magnitude of the sliding offset and the type of interface, it is shown that the average offsets are generally a maximum at the Zn-Zn boundaries, there is less sliding at the Zn-Al interfaces, and the offsets are a minimum at the Al-Al boundaries. In addition, the distributions of the magnitudes of the sliding offsets are similar at both the low and high elongations. It is concluded that grain boundary sliding is an important deformation process in the superplastic Region II and that it remains important even when the elongation is very high. The nature of the results indicates also that experimental observations of the deformation behaviour in superplastic materials at low elongations (up to 50%) provide meaningful information on the behaviour at much higher (superplastic) elongations.

Superplastic forming diffusion bonding process

Abstract: A method is set forth of forming a hollow structure having a varying mass distribution. Two sheets of a diffusion bondable and superplastic formable material are formed with one flat surface and an opposite surface contoured to have a selected mass distribution. A stop-off material is applied between selected non-peripheral portions of each of the flat surfaces whereat diffusion bonding is to be prevented and superplastic forming is to be required. An intermediate flat core sheet may also be present. The flat surfaces are positioned in abutting relation to each other or to the intermediate flat core sheet. The sheets are subjected to diffusion bonding conditions. The sheets are superplastically formed to expand them apart against a die. The invention also provides a method of preinflating contoured sheets prior to superplastically forming them to avoid truss core rupture. Hollow aircraft engine components are formed by the methods.

Internal Stress Superplasticity in 2024 Al-SiC Whisker Reinforced Composites

Abstract: The isothermal and thermal cycling creep behavior of SiC whisker reinforced aluminum composites was investigated. Compression creep tests were performed for two volume fraction composites (10% SiC ...

Heater for superplastic forming of metals

Abstract: A superplastic forming machine having ceramic platens which are oriented to each other with heating elements of each platen arranged at a substantially right angle relationship to the heating elements of the other platen. The heating elements are formed as wires in rectangular sections which overlie each other to permit the increased preferable heating of the areas of greatest heat loss, such as the corners and the periphery of a superplastic metal forming heated platen.

Superplastic deformation behavior in commercial and high purity Zn- 22 Pct Al

Abstract: The superplastic deformation behavior of two grades of the Zn-22 pct Al alloy was studied in detail under identical conditions of grain size, temperature, and stress. While these two grades were prepared by the same procedure, they have different impurity levels; grade 1 is of commercial purity (180 ppm of impurities) and grade 2 is of very high purity (6 ppm of impurities). The experimental results on the commercial grade show that the relationship between the applied stress and steady-state strain rate is sigmoidal and is manifested by the presence of three deformation regions; in region I (low-stress region) the stress exponent,n, is 3.8 and the activation energy for creep,Q, is higher than that for grain boundary diffusion,Q gb; in region II (intermediate-stress region)n = 2.5 andQ ≃ Q gb; and in region III (high-stress region)n again increases. The results on the high purity grade, when compared with those on the commercial grade, reveal a significant difference: the high purity grade, unlike the commercial grade, does not exhibit region I at low stresses. The difference in creep behavior between the two grades of Zn-22 pct Al at low stresses leads to the implication that the origin of region I during superplastic flow is related to the presence of impurity atoms. It is suggested, on the basis of consideration of various impurity-controlled deformation processes, that the creep behavior in region I is most likely a consequence of the existence of a threshold stress which is caused by impurity atom segregation at boundaries.

Elevated temperature tensile properties of powder metallurgy Ni3Al alloyed with chromium and zirconium

Abstract: The tensile properties of hot extruded powders of Ni-24.1Al, Ni-19.1Al-8.5Cr, and Ni-17.4Al-7.9Cr-0.5Zr have been evaluated from room temperature to 1000° C. These powder metallurgy materials have a fine grain size that results in relatively little increase in yield stress with increasing temperature compared to coarse-grained or single-crystal materials. The alloy containing chromium and zirconium shows greatly reduced dynamic embrittlement in the temperature range 600 to 800° C where the unalloyed aluminide exhibits brittle behaviour. The Cr- and Cr + Zr-containing alloys deform superplastically above 900° C. The mechanism of superplastic deformation appears to be predominantly grain-boundary sliding.

Effect of Dispersion of ZrO2 Particles on Creep of Fine-Grained Al2O3

Abstract: The interface-reaction controlled diffusion creep in fine-grained Al2O3 polycrystals was affected by the dispersion state of ZrO2 particles. The ZrO2 particles at grain boundaries reduced the creep rate and increased the activation energy for creep. The stress exponents were two for both pure Al2O3 and ZrO2-containing Al2O3 polycrystals. The pure Al2O3 polycrystals deformed more rapidly than the superplastic ZrO2-toughened Al2O3 at 1250° and 1300°C.

Superplastic forming consolidated rapidly solidified, magnestum base metal alloy powder

Abstract: A complex part composed of rapidly solidified magnesium base metal alloy is produced by superplastic forming at a temperature ranging from 160° C. to 275° C. and at a rate ranging from 0.00021 m/sec. to 0.00001 m/sec., to improve the formability thereof and allow forming to be conducted at lower temperatures. The rapidly solidified magnesium based alloy has a composition consisting essentially of the formula Mg bal Al a Zn b X c , wherein X is at least one element selected from the group consisting of manganese, cerium, neodymium, praseodymium and yttrium, "a" range from 0 to about 15 atom percent, "b" ranges from 0 to about 4 atom percent and "c" ranges from about 0.2 to 3 atom percent, the balance being magnesium and incidental impurities, with the proviso that the sum of aluminum and zinc present ranges from about 2 to 15 atom percent. Such an alloy contains fine grain size and finely dispersed magnesium-, aluminum- rare earth intermetallic phases. When formed, the part exhibits good corrosion resistance together with high ultimate tensile strength and good at room ductility temperature, which properties are, in combination, far superior to those of conventional magnesium alloys. The part is suitable for application as a structural member in helicopters, missiles and air frames where good corrosion resistance in combination with high strength and ductility is important.

Analysis of superplastic metal forming by a finite element method

Abstract: A finite element velocity method for analysing the superplastic sheet metal forming process is presented. This method is developed from the principle of virtual work and is based on the use of isoparametric continuum elements. The large inelastic deformation of the superplastic material is modelled as the behaviour of an incompressible non-linear viscous flow material. The contact and friction problem is solved by using the compatibility load step method, which is an extension of an earlier work. The finite element method is applied to selected problems to illustrate the applicability of the solution procedure.

The mechanical properties of the superplastic AI- 33 Pct Cu eutectic alloy

Abstract: Experiments were conducted to determine the mechanical properties of the superplastic Al-33 Pct Cu eutectic alloy at temperatures from 673 to 723 K. Specimens were tested in a well-annealed condition and there was no evidence for grain growth even at the lowest experimental strain rate of 6.7 × 1(10-7 s-1. It is shown that the stress-strain curves rapidly attain a steady-state value at strain rates below ′10-4 s-1, and there is a sigmoidal relationship between stress and strain rate which may be obtained using several different testing procedures. The maximum elongation to failure recorded in these experiments was 1475 Pct at an initial strain rate of 1.3 × 10-5 s-1. The true activation energy for plastic flow is 175 ±11 kJ mol-1 in the superplastic region II, but it increases to 299 ± 18 kJ mol-1 at low strain rates in region I. The exponent of the inverse grain size is 2.1 ±0.3 in region II. These results show that, when the grains size is stable, there is a genuine region I in the Al-33 Pct Cu alloy at initial strain rates below ∼10-5 s-1.

Principles of superplastic diffusion bonding

Abstract: Superplastic diffusion bonding is now established as an important technique for solid state bonding. There are two types of superplastic diffusion bonding using either transformation or isothermal superplasticity. In this overview, the principles of these two types are summarised and the applications are discussed, including superplastic forming with concurrent diffusion bonding (the SPF-DB process), pressure welding, and the use of superplastic diffusion bonding in sintering.MST/835

Method of treating Ti-Ni shape memory alloy

Abstract: A method of treating a Ti-Ni shape memory alloy to improve their various characteristic properties. In a first step of the method, a wire of the shape memory alloy is held at a high temperature within a predetermined range to be turned into a solid solution, and thereafter, cooled, whereby plastic strain in it is removed and crystals of the alloy are grown. In a second step of the method, current pulse is passed through the wire to rapidly heat it to a temperature higher than its Mf point to cause elongation due to transformation superplasticity to it. In a third step of the method, the application of the pulse is stopped, the wire is rapidly cooled to the temperature of its Mf point or below, and tensile load is applied to the wire immediately after the stop of the application of the pulse to cause elongation to the wire again in a cooling process, and thereafter the load is removed or sufficiently decreases to stop the deformation when the value of m concerning the wire is sharply decreased. Then, the second and the third steps are repeated a required number of times.

High strength, heat resistant aluminum alloys

Abstract: HIGH STRENGTH, HEAT RESISTANT ALUMINUM ALLOYS ABSTRACT OF THE DISCLOSURE The present invention provides high-strength and heat resistant aluminum alloys having a composition represented by the general formula AlaMbLac (wherein Mis at least one metal element selected from the group consisting of Fe, Co, Ni, Cu, Mn and Mo; and a, b and c are atomic percentages falling within the following ranges:65 ? a ? 93, 4 ? b ? 25 and 3 ? c ? 15), the aluminum alloys containing at least 50% by volume of amorphous phase. The aluminum alloys are especially useful as high strength and high heat resistant materials in various applications and, since the aluminum alloys specified above exhibit a superplasticity in the vicinity of their crystallization temperature, they can be readily worked into bulk forms by extrusion, press working or hot forging in the vicinity of the crystallization temperature.

High strength and heat-resistant aluminum-based alloy

Abstract: PURPOSE: To provide the title alloy with high strength so as to endure great bending and with heat resistance by adequately regulating the content of specific metallic elements and specific metallic elements including rare earths to be compounded into an Al-based alloy. CONSTITUTION: An Al-based alloy is formed with the compsn. shown by the general formula of AlaMbXc ((a), (b) and (c) satisfy 50≤a≤95, 0.5≤b≤35 and 0.5≤c≤25); where M denotes one or more kinds of metallic elements selected from V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Ti, Mo, W, Ca, Li, Mg and Si and X denotes one or more kinds of metallic elements selected from Y, La, Ce, Sm, Nd, Hf, Nb, Ta and Mm (misch metal). The structure of the alloy is constituted of amorphous one, of a multiphase body of amorphous one and fine crystal one or of fine crystal one. The Al-based alloy shows superplastic phenomenon near the crystallization temp., and the working such as extruding and pressing can be executed thereto. COPYRIGHT: (C)1989,JPO&Japio